Scientific American: The two most common ways of generating power from the sun both have their drawbacks. Photovoltaic cells, which absorb photons from sunlight and convert them to electricity, operate with only 20 percent efficiency. That is because they can use only photons within a certain range of wavelengths to excite electrons. Solar-thermal systems, which turn sunlight into heat and then into electricity, are more efficient than photovoltaics--because they can use the entire solar spectrum, they can reach efficiencies...